1. Field of the Invention
This invention relates to optoelectronic components, and more particularly, to a waveguide-based optically absorbing device.
2. Description of the Related Art
(Note: This application references a number of different patents, applications and/or publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different patents, applications and/or publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these patents, applications and/or publications is incorporated by reference herein.)
Photonic devices that absorb laser light play a critical role in modem high speed optical transmission systems. Examples of such devices include electro-absorption modulators, waveguide photodetectors, and semiconductor Mach-Zender modulators. In these devices, an electric field is applied across a waveguide layer to change the absorption characteristics of the semiconductor material. Normally, the waveguide is embedded in a pn-junction of semiconductor material to apply the field and is itself either undoped or slightly p-type or n-type doped. The waveguiding properties of the device are typically controlled by fashioning the upper cladding layer into a narrow ridge. The modulators and photodetectors are sometimes monolithically integrated with laser diodes, or widely tunable laser diodes. [1], [2].
If the waveguide layer is reasonably thick, the Franz-Keldysh effect causes the change in absorption with electric field. In the case of quantum well material, the Quantum Confined Stark effect causes the change in the absorption behavior. In both cases, the electric field causes an increased absorption for wavelengths below the bandgap energy. The amount of absorption increase depends on the applied electric field strength and the energy separation of the incoming light to the bandgap energy of the semiconductor.
A constant absorption coefficient in the waveguide layer will cause the light intensity to decrease in an exponential way as the light progresses through the device. The highest amount of light absorption therefore occurs at the front of the device, where the light intensity is highest. The absorbed light generates a photo-induced current that causes a local heating of the device. Because the light intensity decreases along the device, so does the photocurrent, and therefore also the local heating. Thus, the heating is the strongest at the front of the device structure.
In a simple device design, the heat that is generated in the waveguide layer is dissipated predominantly into the substrate. The thermal resistance between the waveguide section and the substrate determines the magnitude of the temperature increase resulting from the dissipated electrical power. Since the dissipated power is highest at the front of the device, the temperature rise is also the highest at the front and decreases along the length of the device. Strong heating at the front of the device will cause a local increase of the absorption coefficient of the semiconductor material due to bandgap shrinkage. This increase of the absorption coefficient further increases the amount of light absorbed per unit length, creating a positive feedback effect that can significantly enhance the local temperature rise at the front of the device. [3] This feedback cycle can cause a thermal runaway, resulting in a catastrophic device failure. [4] Even if the device does not fail immediately, high local temperatures during operation can negatively impact the long term device reliability. A device design with a more evenly distributed light absorption would be highly desirable. This would result in a temperature profile that is not as strongly peaked at the device input, and in a lower peak temperature for a given optical input power. Since the peak temperature limits the power handling capability of the device, such a design would also extend the power capability of the photodetector or modulator device under consideration. [5]
One approach to reduce the peak temperature is to use a thick metal contact layer as a heat spreader. A similar approach has been employed in vertical-cavity surface-emitting laser diodes (VCSELs) to decrease the thermal resistance effectively. [6] For the waveguide photodetectors and modulators under consideration, the use of thick metal will help to reduce the peak temperature somewhat by enhancing heat flow away from the point of maximum generation at the input of the device. Nonetheless, values of the local temperature rise under operation can remain quite high, and the heat generation remains peaked at the device input.
Distributed absorption devices have been investigated in the past for photodetector applications. These investigations were primarily motivated by the desire to reduce peak absorption in order to avoid carrier-screening effects that can compromise device linearity. However, distributed absorbers also help to reduce temperature peaking. [7] S. Jasmin, et. al., have demonstrated a device in which the waveguide geometry is varied along the longitudinal axis to control the optical confinement factor, and therefore the optical absorption. [8] Other researchers have relied on using multiple separate waveguide photodetectors, grouped in series or in parallel, that are connected by a velocity-matching electrode structure. Techniques used to control absorption in the different detectors have included fabricating multimode interferometers to split the light into parallel optical paths to feed separate detectors [9], or connecting series photodetectors via a resistor network to limit the photocurrent produced in each section. [10]
These designs all add extra fabrication complexity to the device. In addition, many of the structures are only compatible with a velocity-matching traveling-wave electrode structure. In contrast, the present invention comprises a novel method for heat reduction through segmented absorption within a single device structure. The fabrication complexity is kept to a minimum, and the device is compatible with either lumped-electrode or velocity-matched electrode configurations.